Its purpose is to: a) extend the observation and measurement of the effect of systemic venous pressure elevation to other tissues vasculature particularly of striated (cremaster) muscle. b) determine from pressure and diameter data, wall tension change resulting in arterioles and sphincter constriction following venous pressure elevation. c) electrical stimulation of selected vasoactive sites of the CNS and medula in acute preparations will serve to examine the extent of microvascular response and to assess the interaction of selected naturally occurring mediators with neurotransmitter (s); extend the above study to the lymphatic vessels. d) increase observation and measurement of wall and lumen dimensions and area in cross-section of arteriolar microvessles in spontaneous hypertensive rats and aging rats. e) the effort will be made to refine marker injection and define mean transit time (MTT) from penetrating arteriole to emerging venule in the brain of laboratory animals, under known depth of anesthesia.